I. Field of the Invention
This invention relates to an optical recording device such as a laser printer.
II. Description of the Prior Art
Optical recording devices using a laser beam can be roughly divided into a group of optical mode recording devices and a group of heat mode recording devices. Optical mode recording devices use either a photo-sensitive material employed in an electrophotographic recording system or an ordinary silver salt photo-sensitive material as a recording material. A laser beam is used to optically form an image on the recording material. The heat mode recording devices use a heat-sensitive sheet or a thermal transfer sheet as a recording material. A laser beam is used to thermally form an image onto the recording sheet.
FIG. 10 is an explanatory diagram outlining one example of a conventional optical recording device using an optical mode electrophotographic recording system.
As shown in FIG. 10, an optical recording device including a drum-shaped photo-sensitive body 1 is turned in the direction of the arrow as its surface is uniformly charged by a charging unit 2. A video signal generating circuit 3 supplies a video signal 4 to an optical modulator 5 with predetermined timing. A laser beam generator 6 transmits a laser beam 7 to the optical modulator 5, so that the laser beam 7 is modulated according to the video signal 4. The modulated laser beam 7 passes through a lens 8 to a scanning means such as polygon mirror 10 which is turned at a constant speed by a motor 9. Polygon mirror 10 includes a series of mirrors which are arranged to constitute the sides of a polygon. The laser beam reflected from the polygon mirror 10 performs the main scanning of the photo-sensitive body 1 by horizontally scanning the charged picture region of the photo-sensitive body 1.
The auxiliary scanning is achieved by the rotation of the photo-sensitive body 1 in the direction of the arrow. Such rotation is preferably made in a step-wise fashion by a stepping motor. Thus, an electrostatic latent image corresponding to the video signal 4 is formed in the picture region of the photo-sensitive body 1. The latent image is developed with toner when passing through a developing unit 11 so that a toner image is formed on the photo-sensitive body 1. Therefore, photo-sensitive body 1 constitutes a means for receiving the line-scanned beam in the form of a scanned spot and for recording an image formed by the scanned spot.
A recording sheet 12 supplied from a sheet supplying tray (not shown) is delivered between the photo-sensitive body 1 and a transferring unit 13 with predetermined timing so that the toner image on the photo-sensitive body 1 is transferred onto the recording sheet 12. The sheet 12 is delivered to a fixing unit 14, where the image on the sheet 12 is fixed. Then the treated sheet is sent to an ejecting tray (not shown). The toner remaining on the surface of the photo-sensitive body 1 is discharged by a discharging unit 15 and removed by a cleaning unit 16.
When an electronic copying machine uses a scan unit comprising lenses according to an image magnifying factor, a copying operation is performed by directly increasing or decreasing the size of the image of an original. Therefore, when the image as a whole is increased or decreased in size, dots and lines forming the image are also increased or decreased in size. Accordingly, when the image thus changed in size is copied, the resultant image is completely similar to the image of the original. In other words, the copied image is a realistic copy.
However, in the optical recording device described above, the laser beam 7 is so designed that its spot size is uniform in the range of from 25 um to 75 um. When an image is recorded, and its size is increased or decreased, although the image as a whole is increased or decreased in size, the size of the dots forming the image remain unchanged. Accordingly, the resultant image is not completely similar to the original image. In other words, the copied image is not a realistic copy.